1. Field of the Invention
The invention relates to thermoplastic elastomer compositions based on a blend of a thermoplastic material and a cured or non-cured elastomer, and to the processing of such compositions.
2. Description of Related Art
A thermoplastic elastomer is generally defined as a polymer or blend of polymers that can be processed and recycled in the same way as a conventional thermoplastic material, yet has properties and performance similar to that of vulcanized rubber at service temperatures. Blends or alloys of plastic and elastomeric rubber have become increasingly important in the production of high performance thermoplastic elastomers, particularly for the replacement of thermoset rubber in various applications.
Polymer blends which have a combination of both thermoplastic and elastic properties are generally obtained by combining a thermoplastic polymer with an elastomeric composition in a way such that the elastomer is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. Early work with vulcanized compositions is found in U.S. Pat. No. 3,037,954 which discloses static vulcanization as well as the technique of dynamic vulcanization wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer is cured while continuously mixing and shearing the polymer blend. The resulting composition is a microgel dispersion of cured elastomer, such as butyl rubber, chlorinated butyl rubber, polybutadiene or polyisoprene in an uncured matrix of thermoplastic polymer such as polypropylene.
Depending on the ultimate application, such thermoplastic elastomer (TPE) compositions may comprise one or a mixture of thermoplastic materials such as propylene homopolymers and propylene copolymers and like thermoplastics used in combination with one or a mixture of cured or non-cured elastomers such as ethylene/propylene rubber, EPDM rubber, diolefin rubber, butyl rubber or similar elastomers. TPE compositions may also be prepared where the thermoplastic material used is an engineering resin having good high temperature properties, such as a polyamide or a polyester, used in combination with a cured or non-cured elastomer. Examples of such TPE compositions and methods of processing such compositions, including methods of dynamic vulcanization, may be found in U.S. Pat. Nos. 4,130,534, 4,130,535, 4,594,390, 5,177,147 and 5,290,886, as well as in WO 92/02582.
TPE compositions are normally melt processed using conventional thermoplastic molding equipment such as by injection molding, compression molding, extrusion, blow molding or other thermoforming techniques. In such TPE compositions, the presence of the elastomeric component does not necessarily improve the processability of the composition. In fact, where the elastomeric component is partially or fully cured (cross-linked) in-situ during the mixing of the TPE polymer components (dynamically vulcanized), or where a dynamically vulcanized TPE composition is further processed, there are heavier demands placed upon processing machinery as compared with the processing of a thermoplastic composition which is free of cured elastomer. Increases such as higher motor load, head pressure and/or torque can place undesirable, unacceptable, or unattainable requirements on specific machinery. For instance, a specific extruder having a specific motor power and gearing, will reach a maximum of motor load, or head pressure, under certain melt temperature conditions for a given polymer being processed. If a polymer or polymer blend is introduced to such an extruder which has such a higher requirement for power to process at least one component, such as a polymer having higher molecular weight and/or narrower molecular weight distribution and/or lower shear sensitivity, the extruder will reach a maximum of one or several of these parameters, and be therefore limited in its ability to pump/perform at a similar level to the performance expected with a more easily processable polymer. In the alternative, if melt blending or processing machinery is to be used for certain production/extrusion, and it is not so limited, the prospect of using more power or increasing head pressure for a more difficult to extrude material would be achievable, but the user of the machinery would still nonetheless desire to conserve power.
Additionally, TPE compositions may exhibit other imperfections during extrusion, specifically film extrusion, that may be undesirable, such as melt fracture. These imperfections are undesirable from a quality standpoint. For example, melt fracture also known as "shark skin" or "orange peel", can lead to poorer optical properties and/or diminished film physical properties that are generally unacceptable. Adjustments to the extrusion process which are made to avoid the development of melt fracture generally involve a slowing down of the process which leads to a reduced rate of extrudate output.
Various prior art references generally disclose the addition of various additives to olefin polymer compositions to improve the extrusion or other properties of the polymer. For example, GB 1,104,662 teaches addition of the salt of alkyl benzene sulfonic acids to polyolefins that purportedly gives a beneficial effect on melt extrusion behavior of the polyolefin. The purported effect is the reduction of the occurrence of "shark skin" or "orange peel". Both alkali and alkaline earth metal salts of alkyl benzene sulfonic acids are said to be effective.
GB 1,078,738 discloses that addition of an "external lubricant" to high molecular weight polyolefins can, purportedly, reduce occurrence of melt fracture. Suggested as external lubricants are salts of monovalent to tetravalent metals, and saturated or unsaturated carboxylic acids containing 10 to 50 carbon atoms. Sulfonates corresponding to the fatty acid salts are also said to be suitable.
JP A 59-176339 discloses that when polyolefins are narrowed in MWD or given higher molecular weight, poor fluidity results which in turn gives rise to melt fracture. The solution suggested is addition of fluorinated compounds including potassium salts of fluoroalkylsulfonic acids. These potassium salts are said to exhibit preferable temperature dependence when compared to other cations such as sodium, calcium, lithium and ammonium. The polyolefin/salt combination is said to be effective at 230.degree. C. or higher.
DE 2,823,507 discloses calendered ethylene polymers and propylene polymers containing alkali or alkaline earth mono sulfonates such as alkyl sulfonates, alenyl sulfonates, alkylaryl sulfonates and succinic acid dialkyl ester sulfonates. Sodium or calcium mono sulfonates are preferred. A suggested benefit is purported to be outstanding separation of the polymer from calendering rolls.
Canadian Patent 731,225 discloses the use of alkali metal salts of certain monosulfonic acids as additives to crystallizable polypropylene compositions to modify the crystallization properties of the polymer. Although these compositions may also include an elastomeric polymer as an impact modifier, the reference does not indicate that the additives improve polymer processability.
There is a need therefore for a relatively inexpensive, easily implemented solution to the processing problems outlined above. Such a solution should also include a material that will readily melt or incorporate into the melted TPE and not adversely affect physical properties, not interfere with crosslinking chemistry or structure produced by that chemistry, not be extractable, or negatively impact organolleptics of shaped TPE articles. Specifically, there is a commercial need for a material that may be easily incorporated into TPE compositions, that will reduce or eliminate the increased power requirement (e.g., motor load and or torque) and increased head pressure.